(MSVC 2017) Buildfix

Makefile.libretro

@@ -454,7 +454,7 @@ else ifneq (,$(findstring windows_msvc2017,$(platform)))
 	reg_query = $(call filter_out2,$(subst $2,,$(shell reg query "$2" -v "$1" 2>nul)))
 	fix_path = $(subst $(SPACE),\ ,$(subst \,/,$1))
 
-	ProgramFiles86w := $(shell cmd /c "echo %PROGRAMFILES(x86)%")
+	ProgramFiles86w := $(shell cmd //c "echo %PROGRAMFILES(x86)%")
 	ProgramFiles86 := $(shell cygpath "$(ProgramFiles86w)")
 
 	WindowsSdkDir ?= $(call reg_query,InstallationFolder,HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Microsoft SDKs\Windows\v10.0)